The use of semiconductor-on-insulator (SOI) wafers in making MOSFET devices has become common in the art. On an SOI wafer, a semiconductor layer is provided which is disposed over a buried oxide (BOX) layer or insulator layer. SOI MOSFET transistors offer improvements over bulk MOSFET transistors in terms of circuit speed, reductions in chip power consumption, and in channel-length scaling. These advantages arise, at least in part, from the decreased junction capacitance made possible by the presence in these devices of a dielectric layer under the active semiconductor region.
The use of a thin layer of strained silicon in the channel layer of MOSFET devices has also been found to improve the performance characteristics of these devices. The presence of strain in the channel layer causes the individual silicon atoms within that layer to be forced farther apart or closer together in their lattice structure than would be the case in the unstrained material. The larger or smaller lattice spacing results in a change in the electronic band structure of the device such that current carriers (i.e., electrons and holes) have higher mobilities within the channel layer, thereby resulting in higher currents in the transistor and faster circuit speeds.
The use of strained silicon channel layers in SOI MOSFETs combines the advantages of a strained silicon channel with the advantages attainable with MOSFET devices. Thus, in SOI MOSFETs, the presence of a buried insulator can drastically reduce parasitic capacitance, while the use of a strained silicon channel in a MOSFET device enhances the drive current of the device.
However, the use of strained silicon channels in SOI MOSFETs offers additional advantages over the use of such channels in bulk MOSFETs. Thus, in bulk MOSFETs, strained silicon channels are typically formed on a thick layer of SiGe, so the source and drain junctions are formed within the SiGe layer. Since SiGe has a lower energy gap and higher dielectric constant, this leads to higher junction capacitances and junction leakage. By contrast, when a strained silicon channel is formed in an SOI structure, the increased junction capacitance and leakage associated with SiGe are mitigated by the SOI structure, and thus are less detrimental to transistor performance.
Despite the aforementioned notable advantages of strained SOI MOSFETs, a number of challenges remain in the implementation of these devices. In particular, SOI MOSFETs frequently exhibit drive currents (Idsat) that are lower than the values which should theoretically be obtainable. There is thus a need in the art for SOI MOSFETs with improved drive currents, and for methods of making such MOSFETs. These and other needs may be addressed by the devices and methodologies disclosed herein.